Windows Updates (WSUS) - Update Classifications.

• Critical updates:
  Broadly released fixes for specific problems addressing critical, non-security related bugs.
• Definition Updates:
  Updates to anti-malware or other definition files.
• Feature Packs:
  New product functionality usually included in the next full product release.
• Security Updates:
  Broadly released fixes for specific products, addressing security issues.
• Service Packs:
  Cumulative sets of all hotfixes, security updates, critical updates, and updates created since the release of the product. Service packs might also contain a limited number of customer-requested design changes or features.
• Tools:
  Utilities or features that aid in accomplishing a task or set of tasks.
• Update Rollups:
  Cumulative sets of hotfixes, security updates, critical updates, and updates packaged together for easy deployment. A rollup generally targets a specific area, such as security, or a specific component, such as Internet Information Services (IIS).
• Updates:
  Broadly released fixes for specific problems addressing non-critical, non-security related bugs.